This invention relates to methods of treatment carried out by increasing nitric oxide levels in cells in need thereof, such as cells that is heterozygous or homozygous for the APOE4 allele.
As the major risk factor for late-onset Alzheimer""s Disease, about 40% of all Alzheimer""s cases are linked to the presence of an APOE4 allele [Saunders, A. M. et al., Neurology 44, 2420-2421 (1994)]. The APOE locus on chromosome 19 encodes three isoforms of apolipoprotein-E whose abundance in healthy populations is approxiatmately 8% APOE2, 78% APOE3, and 14% APOE4 [Mahley, R, Science 240, 622-630 (1988)]. Although the exact biochemical mechanism connecting the presence of APOE4 with the neuronal dysfunction and death seen in the brains of Alzheimer""s patients has yet to be resolved, we hypothesize that apolipoprotein-E (APOE) may function in an isoform-specific manner to regulate cell survival.
Not all apolipoproteins are present in the brain and liver transplant studies show that apolipoprotein-E is made locally in the brain. As the most abundant apoprotein in human brain, APOE is made at low levels by a wide variety of cell types including macrophages and their brain counterparts, microglia [Basu, S., et al., Proc. Natl. Acad. Sci. U.S.A. 78, 7545-7549 (1981); Uchihara, T. et al.,. Neurosci. Letts. 195, 5-8 (1995)]. Cerebrospinal fluid levels of APOE range from 5 to 250 nM where it exists as a mostly xe2x80x9cbarexe2x80x9d apoprotein and as an apoprotein/lipid complex or lipoprotein [Landen, M. et al., Dementia 7, 273-278 (1996)]. In the blood, circulating lipoprotein complexes are classified by their apoprotein content and buoyant density. APOE is most abundant in the very low density lipoproteins particles of the blood [Roheim, P. et al., Proc. Natl. Acad. Sci. U. S. A. 76, 4646-9 (1979)] whose equivalents in the brain are a matter of active investigation. APOE expression is also increased in neurons and astrocytes surrounding the site of brain lesions [Han, S. et al., Neuropathol. Exp. Neurol. 53, 535-544 (1994).]. Recent reports suggest that APOE3-containing lipoproteins support the neurite outgrowth associated with healthy neuronal function to a greater degree than APOE4-containing lipoproteins [Nathan, B. et al., Science 264, 850-852 (1994)]. Consistent with this role, Miyata and Smith [Nature Genetics 14, 55-61 (1996)] recently proposed an antioxidant function for APOE that is isoform-specific in its ability to protect neurons from oxidative challenge. Thus, APOE appears to be present both at the time of cell injury and at the sites of cell injury where it could either serve as part of the mechanism to protect against cell damage or as active participants in cell destruction.
Alzheimer""s is a chronic neurodegenerative disease characterized by neuritic plaques in the patient""s brain which contain apolipoprotein-E, fibrillar amyloid-xcex2eta peptide, dystrophic neurites and activated microglia [Perlmutter, L. et al., Neurosci. Lttrs. 119, 32-36 (1990)]. Like other tissue macrophages, activated microglia release oxyradicals such as superoxide anion and nitric oxide (NO) in response to a variety of factors. Recent evidence, however, demonstrates that the production of oxyradicals by human macrophages significantly differs from those of rat or mouse macrophages [Colton, C. et al., Mol. Chem. Neuropathol. 28, 15-20 (1996)].
Using a human model for microglial function, monocyte-derived macrophages from volunteer donors, we have now directly tested whether APOE modulates NO production. Axcex2 has also been implicated as a toxic agent in Alzheimer""s through a proposed mechanism of NO synthesis and release [Meda, L. et al., Nature 374, 647-650 (1995)]. Since Axcex2 forms stable complexes with APOE [Strittmatter, W. et al., Proc. Natl. Acad. Sci. U.S.A. 90, 8098-8102 (1993)], we have also investigated their effect of Axcex2 alone and in combination with APOE on NO production in human cells.
In view of the foregoing, a first aspect of the invention is, accordingly, a method of treating cells that carry at least one APOE4 allele. The method comprises increasing nitric oxide levels in the cells (e.g., by administering an exogenous source of nitric oxide to the cells) by an amount sufficient to combat the decrease of nitric oxide level associated with the presence of the APOE4 allele.
A second aspect of the invention is a method of increasing nitric oxide levels in cells in need thereof. The method comprises administering APOE to the cells in an amount sufficient to increase nitric oxide levels in the cells.
A third aspect of the present invention is the use of an exogenous nitric oxide source for the preparation of a formulation or medicament for increasing nitric oxide levels in cells.
A fourth aspect of the present invention is the use of APOE for the preparation of a formulation or medicament for increasing nitric oxide levels in cells.
The foregoing and other objects and aspects of the present invention are explained in detail in the drawings herein and the specification set forth below.